Poly(ADP-ribose) polymerases (PARPs) belong to a family of 18 members identified to date, that catalyze the addition of ADP-ribose units to DNA or different acceptor proteins (Ame, J. C., C. Spenlehauer, et al. (2004). Bioessays 26(8): 882-893). PARP-1 is a 113 kDa nuclear protein with three major structural domains: a N-terminal DNA binding domain (DBD) with two zinc fingers, a C-terminal catalytic domain and an auto modification domain. PARP-2 is a nuclear protein shares 68% homology with PARP-1 in its catalytic domain. Among the PARPs, only PARP-1 and PARP-2 are involved in the repair of DNA single strand breaks. PARP-1/2 (PARP-1 and PARP-2) play multiple roles in DNA repair, signal transduction and gene regulation (Kraus, W. L. and J. T. L is (2003). Cell 113(6): 677-683; Ame, J. C., C. Spenlehauer, et al. (2004). Bioessays 26(8): 882-893; Jagtap, P. and C. Szabo (2005). Nat. Rev. Drug Discov. 4(5): 421-440).
PARP-1/2 are recruited to the site of DNA damage and activated upon DNA binding through its zinc fingers, followed by poly (ADP-ribose)ation on histone glutamate residues. This results in a highly negatively charged ADP-ribose chain, which in turn leads to the unwinding and repair of the damaged DNA through the base excision repair mechanism. PARP-1/2 also regulate cell proliferation, differentiation, DNA repair and chromosome stability through interactions with multiple nuclear components such as the nick-sensor DNA ligase III, the adaptor factor XRCC1, DNA polymerase-beta and DNA ligase III, Cockayne syndrome-B protein, Werner syndrome nuclear protein, DNA topoisomerase I activity. PARP-1 is responsible for majority of the DNA damage associated with PARP activity (>90%). Because of its multi-function roles, the activation of PARP-1/2 has been implicated in several human diseases such as cancer, stroke, myocardial infarction, inflammation, hypertension, atherosclerosis and diabetes.
Radiation and chemotherapy are two important therapeutic approaches for cancer treatment. Ionizing radiation and DNA-methylating agents kill cancer cells by mechanisms involving DNA single-strand breaks. However, the single-strand breaks activate PARP-1 and initiate the base excision repair mechanism to repair the damages, which results in reduced potency or drug-resistance. When PARP-1 activity is inhibited, the single strand DNA becomes permanent, which causes genomic dysfunction and apoptosis, eventually cell death (de Murcia, J. M., C. Niedergang, et al. (1997). Proc. Natl. Acad. Sci. U.S.A. 94(14): 7303-7307; Dantzer, F., G. de La Rubia, et al. (2000). Biochemistry 39(25): 7559-7569). Therefore, PARP-1 inhibitor may be used as adjunct anticancer agents to potentiate clinical efficacy of radiation and chemotherapy (de Murcia, J. M., C. Niedergang, et al. (1997). Proc Natl Acad Sci USA 94(14): 7303-7307; Plummer, R., C. Jones, et al. (2008). Clin. Cancer Res. 14(23): 7917-7923).
Breast and ovarian cancers are leading diseases causing death in women. Breast cancer associated gene 1 or 2 (BRCA1 or BRCA2) mutations account for 3-5% of all breast cancers and a greater proportion of ovarian cancers (Wooster, R. and B. L. Weber (2003). N. Engl. J. Med. 348(23): 2339-2347). In “triple negative” breast cancer (deficiency in estrogen receptor a, progesterone receptor expression and HER2 gene), which accounts for approximately 15% of the total breast cancer diagnoses and have a higher likelihood of recurrence and death, the incidence of the mutations are more than 50% (Pal, S. K. and J. Mortimer (2009). Maturitas 63(4): 269-274). BRCA1 and BRCA2 play an integral role in the repair of double-strand breaks in DNA via a mechanism called homologous recombination. BRCA1/2 deficient cells are unable to repair double-strand breaks in DNA, but predominately rely on PARP-1/2 mediated base excision repair to maintain genetic integrity. Inhibition of PARP-1 activity causes synthetic lethality in BRCA1 or 2 mutant cancer cells due to excessive single- and double-strand breaks in DNA, leading to chromosomal aberrations and instability of the genome (Bryant, H. E., N. Schultz, et al. (2005). Nature 434(7035): 913-917; Farmer, H., N. McCabe, et al. (2005). Nature 434(7035): 917-921). Therefore, PARP-1/2 inhibitor may be used as a single agent in treatment of cancers with deficient DNA repair mechanisms (Audeh, M. W., J. Carmichael, et al. (2010). Lancet 376(9737): 245-251; Tutt, A., M. Robson, et al. (2010). Lancet 376(9737): 235-244; O'Shaughnessy, J., C. Osborne, et al. (2011). N. Engl. J. Med. 364(3): 205-214).
PARP-1 activation contributes to various forms of reperfusion injury in brain, heart, kidney, liver and other organs. PARP-1 is excessively activated due to massive damage of DNA and other cellular events under the pathophysiological condition of reperfusion, which leads to rapid consumption of NAD+, depletion of cellular energy, and eventually necrosis. The treatment with PARP inhibitor or PARP1 deficiency reduced infarct size and improved functional outcomes in both brain and heart in preclinical tests (Skaper, S. D. (2003). Ann. N. Y. Acad. Sci. 993: 217-228; discussion 287-288; Szabo, G., L. Liaudet, et al. (2004). Cardiovasc. Res. 61(3): 471-480). PARP-1 inhibitor may be used to treat repufusion diseases such as stroke, and myocardial infarction (Jagtap, P. and C. Szabo (2005). Nat. Rev. Drug Discov. 4(5): 421-440). PARP-1 inhibitors also show pharmacological activity in disease models of inflammation, hypertension, atherosclerosis and diabetic cardiovascular diseases (Jagtap, P. and C. Szabo (2005). Nat. Rev. Drug Discov. 4(5): 421-440).